The definition of a vertical angle is two intersecting lines creating angles that are opposite to one another.
If we apply this criteria to our photo, we can safely assume that the given angle and the angle we are trying to find are vertical angles.
There is a mathematical formula that states vertical angles must be congruent.
Therefore, if we know that our given angle measures 13 degrees, and is vertical angles with the angle we need to find the measure of, we can obtain 13 degrees as our answer.
Therefore, 13 = x.
Hope this helps!